To manufacture sheet-metal products of a predetermined shape by performing punching, bending, drawing, compressing and other types of machining on a sheet-metal blank, such as a steel sheet, the workpiece has heretofore been subjected to several processes. When a large quantity of sheet-metal products is needed, a means for performing several machining processes or stages in a single machining metal die by sequentially feeding the workpiece to the succeeding stages to complete the entire machining process in the final stage has been adopted. This type of multi-stage machining metal die, called the progressive die, has an advantage of high efficiency because one sheet-metal product can be produced with one stamping stroke of the press.
While the conventional type of progressive die, as described above, has advantages of high production rates; short delivery time involved from the charging of a workpiece to the completion of machining and less in-process products; and volume production possible with a small number of workers, it has the following problems. The construction of the metal die becomes extremely complex because a plurality of punch-die sets must be incorporated in a single metal die, requiring a high level of metal-die manufacturing technology, leading to prolonged manufacturing time and increased manufacturing cost.
To replace and repair the damaged metal die, and adjust part of the metal die, the entire metal die has to be disassembled, involving troublesome work, and much time and labor accordingly. Furthermore, in a production system where a wide variety of products are manufactured in a small quantity, specially prepared metal dies have to be manufactured every time the shapes and sizes of workpieces are changed even only slightly. This leads to increased metal-die cost, and makes it difficult to adapt to the so-called flexible manufacturing system (FMS) the need for which has been increasing in recent years.
To solve these problems, the present Applicant has filed a patent application for an index-feed machining system which is simple in construction and can easily perform partial adjustment (Japanese Patent Application Nos. 121760/1990 and 121761/1990, for example). The present invention represents further improvements on these improvement inventions.
FIG. 1 is a perspective view illustrating the essential part of an example of index-feed machining system on which this invention is based. In FIG. 1, numerals 100-500 denote machining units disposed on a base 1 at intervals of 2P (P being a workpiece-feeding pitch) in the direction in which a workpiece (not shown) is fed. A pair of punch and die is provided in each of these machining units 100-500 for a plurality of machining processes. Now, the construction of this invention will be described, taking the machining unit 100 as an example. Numeral 101 denotes a machining unit body formed into an essentially U shape, and having a dovetail 102 integrally provided at the lower end thereof for engaging with a dovetail groove 103 provided on the base 1 so that the machining unit 100 can be adjusted for movement in the workpiece-feeding direction, and at the same time, can be limited in movement in the direction normal to the workpiece-feeding direction. Numeral 104 denotes a movement adjusting device; 105 a clamp; 106 a hydraulic cylinder provided at the upper end of the machining unit body 101; and 107 a position measuring device provided on the side surface of the hydraulic cylinder 106.
Numeral 108 denotes a cassette formed into an essentially U shape and detachably provided on the machining unit body 101, on the upper part of which vertically movably provided is a punch or die (not shown), and on the lower part of which provided is a die or punch (not shown) forming a pair with the aforementioned punch and die. The cassette 108 is positioned by engaging with positioning members 809 and 810, as shown in the machining unit 300 in the figure. Numeral 111 denotes a clamp screw. The cassette 108 is mounted and positioned at a predetermined location on the machining unit body 101 via positioning members (not shown. See numerals 309 and 310 in the machining unit 300.) and securely held in position by tightening the clamp screw 111. After the cassette 108 has been fixedly fitted to the machining unit body 101, the actuator (not shown) of the hydraulic cylinder 106 is connected to the vertically movable punch or die described above.
FIGS. 2A and 2B are diagrams of assistance in explaining the state where a workpiece is machined; FIG. 2A being a plan view and FIG. 2B a cross-sectional view. Like parts are indicated by like numerals shown in FIG. 1. In FIGS. 2A and 2B, numeral 2 denotes a workpiece intermittently fed at a pitch of P in the direction shown by an arrow in the figure. That is, the workpiece 2 is index-fed in a gap between a pair of punch and die provided in the cassette 108 (similarly with other cassettes) in FIG. 1 above. In FIGS. 1 through 2B, the machining units 100-500 are arranged corresponding to the punching process of pilot holes 3, the notching process of arc-segment-shaped notches 4 and the first to third drawing processes.
The machining unit 100 has a punch and die for punching the pilot holes 3, and guides (not shown) engaging with the pilot holes 3 at intervals of P on the downstream side in the direction in which the workpiece 2 is fed. Consequently, as the machining unit 100 is operated, the pilot holes 3 are sequentially punched, and the guides are engaged with the punched pilot holes 3 to prevent the workpiece 2 from unwantedly deviating from the predetermined location thereof, thereby keeping accuracy.
Next, arc-segment-shaped notches 4 are formed in the machining unit 200, the first drawing operation is performed in the machining unit 300 to form a cup-shaped projection 5 on the workpiece 2 while the arc-segment-shaped notches 4 are expanded in width, changing into arc-segment-shaped grooves 6. In the machining unit 400, the second drawing operation and the forming of flange holes 7 are performed, and the height of the projection 5 is increased. The third drawing is performed in the machining unit 500 to further form the projection to a predetermined height. Though not shown in the figures, edge-cutting and other operations are carried out to obtain a sheet-metal product of a predetermined cup shape. Needless to say, positioning is also carried out in the machining units 200-500 by providing guides engaging with the pilot holes 3 to maintain predetermined accuracy.
The index-feed machining system having the aforementioned construction is simple in construction, compared with connectional progressive dies, and easy to manufacture. It has an advantage in that high-efficiency machining can be achieved even in a production system in which a wide variety of products are manufactured in a small quantity, but the following problems are encountered in index-feed machining, including drawing operations.
FIGS. 3A through 3C are cross-sectional diagrams of assistance in explaining the state of drawing operations; FIG. 3A showing the state prior to drawing, FIG. 3B the state in the process of drawing, and FIG. 3C the state after drawing. In FIGS. 3A through 3C, numeral 11denotes a punch, and 12 a die, both corresponding to the machining units 100 and 200 shown in FIGS. 1 and 2A. Next, numeral 13 denotes a drawing die; and 14 a drawing punch, both corresponding to the machining units 300-500 shown in FIGS. 1 and 2A. In the drawing die 13 provided is a knockout pin 16 that is preloaded downward by a spring 15 and formed in a vertically slidable fashion. The drawing punch 14 is fixedly fitted to a retainer plate 17 and has a blank holding pad 18. The blank holding pad 18 is connected to a movable plate 20 via a rod 19 passing through the retainer plate 17 in a vertically movable fashion, and preloaded upwards by a spring 21.
To carry out a drawing operation with the above-mentioned construction, the punch 11 and the drawing die 13 are actuated downward from the state shown in FIG. 3A by the hydraulic cylinder 106 shown in FIG. 1 for example, then drawing is performed as shown in FIG. 3B. That is, punching is performed by engaging the punch 11 with the die 12, and drawing is performed by the drawing die 13 and the drawing punch 14. During drawing, the drawing die 13 and the blank holding pad 18, and the knockout pin 16 and the drawing punch 14 hold the workpiece 2 between them from above and below, then the drawing punch 14 enters in the drawing die 13 to carry out the drawing operation. During this drawing operation, the blank holding pad 18 forces the workpiece 2 onto the lower end face of the drawing die 14 by a predetermined spring pressure. Thus, the workpiece 2 is allowed to be moved horizontally at that location to cause the plastic deformation of the workpiece 2, and prevented from producing unwanted wrinkles. Symbol h denotes drawing depth or the height of the projection 5. Upon completion of drawing operation, the punch 11 and the drawing die 13 are moved upwards by the returning operation of the hydraulic cylinder 106, as shown in FIG. 3C, causing the workpiece 2 to be indexed. At this time, the projection 5 can be easily removed from the drawing die 13 because the knockout pin 16 is preloaded downward by the spring 15.
As is evident from FIG. 3B, the workpiece 2 is pushed down from the level before drawing to the drawing depth h during drawing, deformed between the punch 11 and the drawing die 13, and then returned to the original level after drawing operation, as shown in FIG. 3C. If the workpiece 2 is moved up and down in this way, tensile, compressive and bending stresses are generated in the workpiece 2, resulting in the deformation of products and lowered dimensional accuracy. Increasing the intervals of the machining units to eliminate these problems would increase the size of the entire system, requiring an unwantedly large space for the system.
As is evident from FIGS. 3A through 3C, the drawing die 13 reciprocates a stroke of 2h+.alpha. for drawing operation. Symbol .alpha. used here is a gap set to ensure the smooth index-feeding of the workpiece 2. That is, a stroke of 2h+.alpha. is needed for the drawing die 13 to draw to the drawing depth h. In general, the larger the stroke of a hydraulic cylinder, the larger becomes the required energy. In index-feed machining systems to which this invention is applied, in which a plurality of machining units have independent driving means, the use of the aforementioned hydraulic cylinders as driving means requires 2h+.alpha. or more of stroke in machining means, including machining units succeeding the drawing process. This poses a problem of the increased required volume of operating fluid.
Next, the conventional index-feed machining systems usually handle strip-shaped workpieces, and therefore mostly involve bending, drawing, punching, piercing and other sheet-metal working. As a result, it is difficult to handle certain products incorporating tapped holes, for example, in the index-feed machining process. Such products are therefore manufactured by providing tapped holes separately on the workpiece 2 after the completion of the index-feed machining process. This results in increased cost.
Since the products obtained with index-feed machining are generally of small sizes and are manufactured continuously, the quantity of products in a production lot tends to be large. Providing tapped holes additionally on such a large quantity of products that have already been subjected to the index-feed machining not only requires special-purpose machining jigs, but also additional time and labor for mounting and removing products on the jigs. This poses some problems, such as increased machining cost and the difficulty in improving the dimensional accuracy due to variability in the reference plane.
Furthermore, independent special-purpose hydraulic cylinders 106 and other equipment are provided in a plurality of machining units, as shown in FIG. 1. While this arrangement permits the independent operation of the units and the standardization of common components for interchangeability, if a particular machining unit requires a larger drive force or working load than other units, a hydraulic cylinder of a special specification must be provided for that machining unit. This would not only increase manufacturing cost but also make it difficult to keep balance with other hydraulic cylinders.
Although there can be conceived means for reducing drive force or working load by dividing the particular machining process into multiple steps, this arrangement would increase the number of machining processes, requiring additional machining units to be installed. All this leads to increased cost and system size.
In addition, the operating fluid of hydraulic cylinders is usually maintained at the same pressure, a pressure as high as 140 kg/cm.sup.2, for example In machining the workpiece as described above, however, operating fluid is required to be at a high level only when bending, drawing, punching or piercing operation is performed, but operating fluid need not always be kept operating at high pressure to cause the punch or die to come near or keep away from the workpiece. In hydraulic cylinders, on the other hand, a large amount of energy is required to raise the pressure of operating fluid. Since conventional hydraulic cylinders require high-pressure operating fluid at all times, and involve larger strokes than needed, the required volume of operating fluid is increased, and accordingly energy consumption is increased.